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Outflow and Infall in a Sample of Massive Star-forming RegionsWe present single-pointing observations of SiO, HCO+, andH13CO+ from the James Clerk Maxwell Telescopetoward 23 massive star-forming regions previously known to containmolecular outflows and ultracompact H II regions. We detected SiO toward14 sources and suggest that the nondetections in the other nine sourcescould be due to those outflows being older and without ongoing shocks toreplenish the SiO. We serendipitously detected SO 2 toward 17sources in the same tuning as HCO+. We detectedHCO+ toward all sources, and suggest that it is tracinginfall in nine cases. For seven infall candidates, we estimate massinfall rates between 1×10-2 and 2×10-5Msolar yr-1. Seven sources show both SiOdetections (young outflows) and HCO+ infall signatures. Wealso find that the abundance of H13 CO+ tends toincrease along with the abundance of SiO in sources for which we coulddetermine abundances. We discuss these results with respect to currenttheories of massive star formation via accretion. From this survey, wesuggest that perhaps both models of ionized accretion and haltedaccretion may be important in describing the evolution of a massiveprotostar (or protostars) beyond the formation of an H II region.

Infrared Extinction toward Nearby Star-forming RegionsWe present an independent estimate of the interstellar extinction lawfor the Spitzer IRAC bands, as well as a first attempt at extending thelaw to the 24 μm MIPS band. The source data for these measurementsare observations of five nearby star-forming regions: the Orion A cloud,NGC 2068/2071, NGC 2024/2023, Serpens, and Ophiuchus. Color excessratios EH-Ks/EKs-[λ]were measured for stars without infrared excess dust emission fromcircumstellar disks/envelopes. For four of these five regions, theextinction laws are similar at all wavelengths and differ systematicallyfrom a previous determination of the extinction law, which was dominatedby the diffuse ISM, derived for the IRAC bands. This difference could bedue to the difference in the dust properties of the dense molecularclouds observed here and those of the diffuse ISM. The extinction law atlonger wavelengths toward the Ophiuchus region lies between that to theother four regions studied here and that for the ISM. In addition, weextended our extinction law determination to 24 μm for Serpens andNGC 2068/2071 using Spitzer MIPS data. We compare these results againstseveral ISO extinction law determinations, although in each case thereare assumptions which make absolute comparison uncertain. However, ourwork confirms a relatively flatter extinction curve from 4 to 8 μmthan the previously assumed standard, as noted by all of these recentstudies. The extinction law at 24 μm is consistent with previousmeasurements and models, although there are relatively largeuncertainties.

Embedded Clusters: Laboratories for Star FormationEmbedded clusters are the fundamental units of star formation in ourGalaxy, therefore studying their properties is critical forunderstanding how star formation proceeds on both the local and Galacticscale. We have surveyed embedded clusters in local molecular clouds withFLAMINGOS and are investigating the star forming histories, IMF,structure and evolution of these young clusters. In this presentation, Iwill discuss our results for the clusters in the Rosette and Orion starforming complexes where we find evidence for the evolution of clusterstructure and variations in the low mass end of the initial massfunction.

Hard X-Rays and Fluorescent Iron Emission from the Embedded Infrared Cluster in NGC 2071We present first results of XMM-Newton X-ray observations of theinfrared cluster lying near the NGC 2071 reflection nebula in the OrionB region. This cluster is of interest because it is one of the closestregions known to harbor embedded high-mass stars. We report thediscovery of hard X-ray emission from the dense central NGC 2071-IRsubgroup, which contains at least three high-mass young stellar objects(NGC 2071 IRS 1, IRS 2, and IRS 3). A prominent X-ray source is detectedwithin 1" of the infrared source IRS 1, which is thought to drive apowerful bipolar molecular outflow. The X-ray spectrum of this source isquite unusual compared to the optically thin plasma spectra normallyobserved in young stellar objects (YSOs). The spectrum is characterizedby a hard broadband continuum plus an exceptionally broad emission lineat ~6.4 keV from neutral or near-neutral iron. The fluorescent Fe linelikely originates in cold material near the embedded star (i.e., a diskor envelope) that is irradiated by the hard, heavily absorbed X-raysource.

Infrared Nebulae around Young Stellar ObjectsWe present a K-band atlas of 106 reflection nebulae, 41 of which are newdiscoveries. We observed these nebulae with the University of Hawaii 2.2m telescope in the course of an imaging survey of 197 objects that wereselected as nearby young Class I sources. K-band images andflux-calibrated surface brightness contour plots of each nebula arepresented. We found that the near-IR luminosities and physical sizes ofthe nebulae increase with the bolometric luminosity of the illuminatingsources. Only 22 nebulae, about 10% of these candidate Class I sources,have indications of shocked H2 emission. The great variety ofnebulae that we observed prevented us from classifying them based onmorphology. However, we note that as the spectral index decreases, thecentral star is more frequently visible at K band, and the flux from thecentral star tends to be dominant over the flux from the nebula. Forobjects that have a higher spectral index, most of the K-band flux isfrom the reflection nebula, and the central star is less frequentlyvisible. The nebula around IRAS 05450+0019 has a unique morphology, andwe speculate that it may be an example of a disk shadow being projectedinto the surrounding cloud. We present J-, H-, and K-band images of thisobject with surface brightness contours, as well as its spectral energydistribution from 1.2 to 100 μm.

OMC-1: A Cool Arching Filament in a Hot Gaseous Cavity: Geometry, Kinematics, Magnetic Vectors, and Pressure BalanceWe have assembled data on large (Galactic shell), middle (filament), andsmall (protostar) scales. OMC-1 is embedded in a hot cavity. Here wehave made multiconstituent (gas/dust/field) homogeneous maps of thecentral part of the OMC-1 filament, covering an area of2'×2'. We find here that the verticalfilament has a turbulent pressure about equal to the magnetic pressure;its total pressure roughly equals that in the surrounding medium. Themagnetic field crosses the filament with an estimated strength of300-350 μG in the sky plane. It could be an ``externally wrapped''U-shaped magnetic field going around the filament (due to a large shockfront traveling in the cavity), or else an internally generated``helical'' field (of unknown origin). Near the OMC-1 map center, we mapthe low-velocity outflow near IRc2-source I, with a pattern of bluesouth-southwest gas and red north-northeast gas. Our James Clerk MaxwellTelescope data yield a plane-of-sky field of 2.4-2.6 mG across theoutflow; this strength nearly equals the published Zeeman line-of-sightdata. Farther out, the Zeeman data show a negative magnetic field,possibly suggesting a pole-on dipolar field.

Looking into the cradle: new mid-IR observations of multiple proto-starsContext: The multiplicity rate of the youngest stellar objects isdirectly linked to their formation process, and therefore represents oneof the most critical parameters to constrain theories of star formation.Multiplicities of embedded protostars are, however, not well determinedempirically. Aims: The aim of this work is to study the multiplicity ofthree embedded proto-stellar systems, namely SSV 63, L1551 NE, andL1551-IRS5, through mid-IR imaging. All of them are suspected binary ormultiple systems, but prone to ambiguities in the literature. Methods:We present high spatial resolution mid-infrared observations obtainedwith VISIR at the VLT. Results: For the SSV 63 system, we report thediscovery of a new Class I companion, approx 2.6 arcsec N of SSV63 E,which was never seen in previous near-IR and radio continuum studies.The companion to SSV 63 W, which was detected at near-IR wavelengths, isalso present in the mid-IR regime. In the case of L1551 NE, we haveconfirmed the presence of a companion at 0.5 arcsec NW from the centralstar, as previously reported by Reipurth et al. (2002, AJ, 124, 1045).Finally, we have not detected any companion to L1551-IRS5. However, wefind signs of extended emission around the target. In total, we havedetected three binaries. The SEDs of the primaries are consistent withtheir Class I/flat-spectrum classification. For the secondaries,ancillary data are required to derive their evolutionary status. Conclusions: .The geometrical arrangement of sources in the SSV63 systemstrongly supports the view of hierarchical fragmentation of cloud cores.Based on observations collected at the La Silla-Paranal Observatory,ESO (Chile), with VISIR at Melipal under program ID 076.C-0518(A).

A SCUBA survey of Orion - the low-mass end of the core mass functionWe have re-analysed all of the Submillimetre Common User Bolometer Array(SCUBA) archive data of the Orion star-forming regions. We have puttogether all of the data taken at different times by different groups.Consequently, we have constructed the deepest submillimetre maps ofthese regions ever made. There are four regions that have been mapped:Orion A North and South, and Orion B North and South. We find that twoof the regions, Orion A North and Orion B North, have deeper sensitivityand completeness limits, and contain a larger number of sources, so weconcentrate on these two. We compare the data with archive data from theSpitzer Space Telescope to determine whether or not a core detected inthe submillimetre is pre-stellar in nature. We extract all of thepre-stellar cores from the data and make a histogram of the core masses.This can be compared to the stellar initial mass function (IMF). We findthe high-mass core mass function (CMF) follows a roughly Salpeter-likeslope, just like the IMF, as seen in previous work. Our deeper mapsallow us to see that the CMF turns over at, ~1.3Msolar abouta factor of 4 higher than our completeness limit. This turnover hasnever previously been observed, and is only visible here due to our muchdeeper maps. It mimics the turnover seen in the stellar IMF at~0.1Msolar. The low-mass side of the CMF is a power law withan exponent of, 0.35 +/- 0.2 which is consistent with the low-mass slopeof the young cluster IMF of 0.3 +/- 0.1. This shows that the CMFcontinues to mimic the shape of the IMF all the way down to the lowercompleteness limit of these data at ~0.3Msolar.

Deficit of Wide Binaries in the η Chamaeleontis Young ClusterWe have carried out a sensitive high-resolution imaging survey of starsin the young (6-8 Myr), nearby (97 pc) compact cluster around ηChamaeleontis to search for stellar and substellar companions. Our datawere obtained using the NACO adaptive optics system on the ESO VeryLarge Telescope (VLT). Given its youth and proximity, any substellarcompanions are expected to be luminous, especially in the near-infrared,and thus easier to detect next to their parent stars. Here, we presentVLT NACO adaptive optics imaging with companion detection limits for 17η Cha cluster members, and follow-up VLT ISAAC near-infraredspectroscopy for companion candidates. The widest binary detected is~0.2", corresponding to the projected separation 20 AU, despite oursurvey being sensitive down to substellar companions outside 0.3", andplanetary-mass objects outside 0.5". This implies that the stellarcompanion probability outside 0.3" and the brown dwarf companionprobability outside 0.5" are less than 0.16 with 95% confidence. Wecompare the wide binary frequency of η Cha to that of the similarlyaged TW Hydrae association and estimate the statistical likelihood thatthe wide binary probability is equal in both groups to be less than2×10-4. Even though the η Cha cluster is relativelydense, stellar encounters in its present configuration cannot accountfor the relative deficit of wide binaries. We thus conclude that thedifference in wide binary probability in these two groups providesstrong evidence for multiplicity properties being dependent onenvironment. In two appendices we derive the projected separationprobability distribution for binaries, used to constrain physicalseparations from observed projected separations, and summarizestatistical tools useful for multiplicity studies.

Interferometric Mapping of Magnetic Fields: NGC 2071IRWe present polarization maps of NGC 2071IR from thermal dust emission at1.3 mm and from CO J=2-->1 line emission. The observations wereobtained using the Berkeley-Illinois-Maryland Association (BIMA) arrayin the period 2002-2004. We detected dust and line polarized emissionfrom NGC 2071IR that we used to constrain the morphology of the magneticfield. From CO J=2-->1 polarized emission we found evidence for amagnetic field in the powerful bipolar outflow present in this region.We calculated a visual extinction Av~26 mag from our dustobservations. This result, when compared with early single-dish work,seems to show that dust grains emit polarized radiation efficiently athigher densities than previously thought. Mechanical alignment by theoutflow is proposed to explain the polarization pattern observed in NGC2071IR, which is consistent with the observed flattening in this source.

A Unified Model for Bipolar Outflows from Young StarsWe develop a unified model for molecular outflows in star formation. Themodel incorporates essential features expected of the primary wind,which is thought to be driven magnetocentrifugally from close to thecentral stellar object, and the ambient core material shaped byanisotropic magnetic support. The primary wind is modeled as atoroidally magnetized fast outflow moving radially away from the origin,with an angle-dependent density distribution: a dense axial jetsurrounded by a more tenuous wide-angle wind, as expected in the X-windmodel. If dynamically significant magnetic fields are present, thestar-forming core will settle faster along the field lines than across,forming a toroid-like structure. We approximate the structure with asingular isothermal toroid whose density distribution can be obtainedanalytically. The interaction of the laterally stratified wind and theambient toroid is followed using the Zeus2D magnetohydrodynamics (MHD)code. We find that the lobes produced by the interaction resemble manysystematics observed in molecular outflows from very young stars,ranging from Class 0 to I sources. In particular, both the dense axialjet and the wide-angle wind participate in the wind-ambient interaction.In our model, the jet- and wind-driven pictures of molecular outflowsare unified. We discuss the observational implications of the unifiedpicture, including the possibility of detecting the primary jet/winddirectly.

Who is eating the outflow? High-angular resolution study of an intermediate-mass protostar in L1206Context: .Up to now only a few intermediate-mass molecular outflows havebeen studied with enough high-angular resolution. Aims: .The aimof this work is to study in detail the intermediate-mass YSO IRAS22272+6358A, which is embedded in L1206, and its molecular outflow, toinvestigate the interaction of the outflow with the dense protostellarmaterial and to compare their properties with those of lower masscounterparts. Methods: .We carried out OVRO observations of the2.7 mm continuum emission, CO (J=1→0), C18O(J=1→0),and HC3N (J=12→11) to map the core of L1206 with high-angularresolution and to derive the properties of the dust emission, themolecular outflow, and the dense protostellar envelope. Results:.The 2.7 mm continuum emission has been resolved into four sources,labeled OVRO 1, 2, 3, and 4. The intermediate-mass Class 0/I object OVRO2, with a mass traced by the dust emission of 14.2 Mȯ,is the source associated with IRAS 22272+6358A. The CO (J=1→0)observations have revealed a very collimated outflow driven by OVRO 2,at a PA ≃ 140°, that has a very weak southeastern red lobe anda much stronger northwestern blue lobe. Photodissociation toward the redlobe produced by the ionization front coming from the bright-rimmeddiffuse Hii region could be responsible for the morphology of theoutflow. The spatial correlation between the outflow and the elongateddense protostellar material traced by HC3N (J=12→11) suggests aninteraction between the molecular outflow and the protostellar envelope.Shocks produced by the molecular outflow, and possibly by the shockfront preceding the ionization front, could account for the southernenhancement of HC3N. The properties of the intermediate-mass protostarOVRO 2 and the molecular outflow are consistent with those of lower masscounterparts. The C18O abundance relative to molecularhydrogen estimated toward OVRO 2 is 3×10-8, a value ~6to 13 times lower than typical abundances estimated toward molecularclouds. The most plausible explanation for such a difference is COdepletion toward OVRO 2.

Methods for analysing structure in molecular cloudsWe have previously reported a dimensionless measure, , which can bothquantify, and distinguish between, the extent to which a star cluster iscentrally concentrated, and the extent to which it contains small-scalesubclusters. is the ratio of the normalized correlation length, , (i.e.the mean projected separation between stars, divided by the overallradius of the cluster), to the mean length, , of the segments of aminimal spanning tree (MST) joining all star positions: .In this paper, we attempt to adapt the correlation-length method to thecharacterization of gas clouds, with a view to comparing directly thestructures of gas clouds and star clusters. We also compare the resultsof the correlation-length method with fractal dimensions estimated usingthe more familiar perimeter-area method whereby the lengths of closedcontours are plotted against the areas they enclose, on a log-log plot.We find that the normalized correlation length, when modified to dealwith pixellated grey-scale data, is a robust indicator of either centralconcentration or fractal subclustering of gas clouds, but cannotdistinguish between the two types of structure. It is, however,extremely reliable, easy to implement and works accurately at all scalesand over all dynamic ranges, even with poorly sampled data. Itimplicitly incorporates edge effects, so all the data in the completecloud are used, and it therefore provides a useful method for comparingthe structures of molecular clouds and star clusters.The normalized correlation length produces comparable results to theperimeter-area method when used on molecular cloud data. However, theperimeter-area method is unable to distinguish the degree of clusteringin three-dimensional objects with fractal dimensions greater than 2.0.It also suffers from measurement noise and lack of objectivity,particularly if only a few contours are selected for analysis. It cannotbe used to compare clouds with star clusters.It is not found possible to construct an MST algorithm which worksreliably for grey-scale data and is immune to scaling problems. Thepreviously reported parameter is therefore not useful when consideringgas clouds.

The Southern Flanking Fields of the 25 Orionis GroupThe stellar group surrounding the Be (B1 Vpe) star 25 Ori was discoveredto be a pre-main-sequence (PMS) population by the CIDA VariabilitySurvey of Orion and subsequent spectroscopy. We analyze Sloan DigitalSky Survey multiepoch photometry to map the southern extent of the 25Ori group and characterize its PMS population. We compare this group tothe neighboring Orion OB1a and OB1b subassociations and to active starformation sites (NGC 2068/NGC 2071) within the LDN 1630 dark cloud. Wefind that the 25 Ori group has a radius of 1.4d, corresponding to 8-11pc at the distances of Orion OB1a and OB1b. Given that thecharacteristic sizes of young open clusters are a few parsecs or less,this suggests that 25 Ori is an unbound association rather than an opencluster. Due to its PMS population having a low classical T Tauri starfraction (~10%), we conclude that the 25 Ori group is of comparable ageto the 11 Myr Orion OB1a subassociation.

CCS and NH3 Emission Associated with Low-Mass Young Stellar ObjectsIn this work we present a sensitive and systematic single-dish survey ofCCS emission (complemented with ammonia observations) at 1 cm, toward asample of low- and intermediate-mass young star-forming regions known toharbor water maser emission, made with NASA's 70 m antenna at Robledo deChavela, Spain. Out of the 40 star-forming regions surveyed in the CCS(21-10) line, only six low-mass sources show CCSemission: one transitional object between the prestellar andprotostellar Class 0 phase (GF9-2), three Class 0 protostars(L1448-IRS3, L1448C, and B1-IRS), a Class I source (L1251A), and a youngT Tauri star (NGC 2071 North). Since CCS is considered an ``early-time''(

An observational survey of molecular emission ahead of Herbig-Haro objectsContext: .A molecular survey recently performed ahead of HH 2 supportsthe idea that the observed molecular enhancement is due to UV radiationfrom the HH object.Aims.The aim of the present work is to determinewhether all HH objects with enhanced HCO+ emission ahead ofthem also exhibit the same enhanced chemistry as HH 2. We thus observedseveral molecular lines at several positions ahead of five Herbig-Haroobjects where enhanced HCO+ emission was previouslyobserved.Methods.We mapped the five Herbig-Haro objects using theIRAM-30 m. For each position we searched for more than one molecularspecies, and where possible for more than one transition per species. Wethen estimated the averaged beam column densities for all speciesobserved and also performed LVG analyses to constrain the physicalproperties of the gas.Results.The chemically richest quiescent gas isfound ahead of the HH 7-11 complex, in particular at the HH 7-11 Aposition. In some regions we also detected a high velocity gascomponent. We find that the gas densities are always higher than thosetypical of a molecular cloud while the derived temperatures are alwaysquite low, ranging from 10 to 25 K. The emission of most species seemsto be enhanced with respect to that of a typical dense clump, probablydue to the exposure to a high UV radiation from the HH objects. Chemicaldifferentiation among the positions is also observed. We attempt a verysimple chemical analysis to explain such differentiation.

A search for water masers toward extrasolar planetsContext: .Water is the most common triatomic molecule in the universeand the basis of life on Earth. Astrophysical masers have been widelystudied in recent years and have been shown to be invaluable probes ofthe details of the environment in which they are found. Water masers,for instance, are often detected toward low-mass star-forming regions.Doppler radial-velocity surveys have detected about 160exoplanets.Aims.Observations of water masers from exoplanetary systemswould give us a new detailed window through which to explorethem.Methods.We present a search for water masers toward eighteenextrasolar planets using the newly upgraded Australia Telescope CompactArray at 12 mm. A sensitivity of 25 mJy beam-1 and anangular resolution of ~10'' were achieved at 22.235 GHz. Results.Nomaser lines are clearly observed.

Large Area Mapping at 850 μm. IV. Analysis of the Clump Distribution in the Orion B South Molecular CloudWe present results from a survey of a 1300 arcmin2 region ofthe Orion B South molecular cloud, including NGC 2024, NGC 2023, and theHorsehead Nebula (B33), obtained using the Submillimetre Common-UserBolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT).Submillimeter continuum observations at 450 and 850 μm are discussed.Using an automated algorithm, 57 discrete emission features (``clumps'')are identified in the 850 μm map. The physical conditions withinthese clumps are investigated under the assumption that the objects arein quasi-hydrostatic equilibrium. The best-fit dust temperature for theclumps is found to be Td=18+/-4 K, with the exception ofthose associated with the few known far-infrared sources residing in NGC2024. The latter internally heated sources are found to be much warmer.In the region surrounding NGC 2023, the clump dust temperatures agreewith clump gas temperatures determined from molecular line excitationmeasurements of the CO molecule. The bounding pressure on the clumpslies in the range log(k-1P cm3K-1)=6.1+/-0.3. The cumulative mass distribution is steep atthe high-mass end, as is the stellar initial mass function. Thedistribution flattens significantly at lower masses, with a turnoveraround 3-10 Msolar.

Photometric Accretion Signatures Near the Substellar BoundaryMultiepoch imaging of the Orion equatorial region by the Sloan DigitalSky Survey has revealed that significant variability in the bluecontinuum persists into the late-M spectral types, indicating thatmagnetospheric accretion processes occur below the substellar boundaryin the Orion OB1 association. We investigate the strength of theaccretion-related continuum veiling by comparing the reddening-invariantcolors of the most highly variable stars against those of main-sequenceM dwarfs and evolutionary models. A gradual decrease in the g-bandveiling is seen for the cooler and less massive members, as expected fora declining accretion rate with decreasing mass. We also see evidencethat the temperature of the accretion shock decreases in the very lowmass regime, reflecting a reduction in the energy flux carried by theaccretion columns. We find that the near-IR excess attributed tocircumstellar disk thermal emission drops rapidly for spectral typeslater than M4. This is likely due to the decrease in color contrastbetween the disk and the cooler stellar photosphere. Since accretion,which requires a substantial stellar magnetic field and the presence ofa circumstellar disk, is inferred for masses down to 0.05Msolar, we surmise that brown dwarfs and low-mass stars sharea common mode of formation.

Multi-Epoch VERA Observations of -0.1We report on multi-epoch observations of H2O maser emissionin the star-forming region OH 43.8‑0.1, carried out with VLBIExploration of Radio Astrometry. The large-scale maser distributionsobtained by single-beam VLBI mapping reveal new maser spots scatteredover the area of 0''.7 × 1''.0, in addition to a `shell-like'structure with a scale of 0''.3 × 0''.5, which was mappedpreviously. Proper motions were also obtained for 43 spots based on5-epoch monitoring with a time span of 281 days. The distributions ofthe proper motions show a systematic outflow in the north-southdirection with an expansion velocity of ˜ 8 km s-1. Theoverall distributions of the maser spots as well as the proper motionsare better represented by a bipolar flow plus a central maser clusterwith a complex structure, rather than a shell with uniform expansion,such as those found in Cep A R5 and W75N VLA2. The distance to OH43.8‑0.1 was also estimated based on the statistical parallax,yielding D = 2.8 ± 0.5 kpc. This distance is consistent with anear kinematic distance, and rules out a far kinematic distance (˜9kpc). Also, the radial velocity of the OH 43.8‑0.1 combined withthe distance provides a constraint on the flatness of the galacticrotation curve, indicating that there is no systematic difference in therotation speeds at the Sun and at the position of OH 43.8‑0.1,which is located at a galacto-centric radius of ˜ 6.3 kpc.

The molecular condensations ahead of Herbig-Haro objects. III. Radiative and dynamical perturbations of the HH 2 condensationWe have carried out an extensive observational study (from BIMA data)and made a preliminary theoretical investigation of the molecular gasaround HH 2. The molecular maps show a very complex morphological,kinematical and chemical structure. For clarity we divided the observedregion in four subregions: (1) The Ahead Core, located ahead of HH 2:its chemistry may be a consequence of a weak UV field originating in HH2. The chemical structure within the Ahead Core suggests that it is nothomogeneous but probably composed of small clumps; (2) The SO2 Clump,which is a molecular component within the Ahead Core that is moreexposed to the UV radiation from HH 2. An increase of density andrelative molecular abundances is observed towards HH 2. The UV radiationis possibly the source of molecular enhancement. Our chemical analysisconfirms that this clump must have substructure within it; (3) the WestCore, which is surrounded by a ring structure of shocked ionized gas andmid-IR emission. The ring structure is likely a consequence of the factthat the core is in the foreground with respect to the shocked and hotcomponent. The chemistry of this core can be best explained as arisingfrom a combination of an old photo-processed dense clump and a PDR, withor without a warm interface created in the interaction of the outflowwith the core; (4) The High Velocity Region, associated with HH 2, istraced by HCO+ but not by other molecular shock tracers. Thechemistry can be accounted for by the interaction of the VLA 1 outflowwith a dense clump via non-dissociative shocks and by the presence of avery strong UV field. The overall main conclusion of this work confirmsthe findings of Papers I and II, by demonstrating that in addition tothe strong photochemical effects caused by penetration of the UV photonsfrom HH 2 into molecular cloud, a range of complex radiative anddynamical interactions occur. Thus, despite the apparent "quiescent"nature of the molecular cloud ahead of HH 2, the kinematical propertiesobserved within the field of view suggest that it is possibly beingdriven out by the powerful winds from the VLA 1 protostar.

How to identify the youngest protostarsWe study the transition from a prestellar core to a Class 0 protostar,using SPH to simulate the dynamical evolution, and a Monte Carloradiative transfer code to generate the SED and isophotal maps. For aprestellar core illuminated by the standard interstellar radiationfield, the luminosity is low and the SED peaks at ~190 μ m. Once aprotostar has formed, the luminosity rises (due to a growingcontribution from accretion onto the protostar) and the peak of the SEDshifts to shorter wavelengths (80~to 100 μ m). However, by the end ofthe Class 0 phase, the accretion rate is falling, the luminosity hasdecreased, and the peak of the SED shifts back towards longerwavelengths (90 to 150 μ m). In our simulations, the density ofmaterial around the protostar remains sufficiently high well into theClass 0 phase that the protostar only becomes visible in the NIR if itis displaced from the centre dynamically. Raw submm/mm maps of Class 0protostars tend to be much more centrally condensed than those ofprestellar cores. However, when convolved with a typical telescope beam,the difference in central concentration is less marked, although theClass 0 protostars appear more circular. Our results suggest that, if acore is deemed to be prestellar on the basis of having no associatedIRAS source, no cm radio emission, and no outflow, but it has a circularappearance and an SED which peaks at wavelengths below ~170 μ m, itmay well contain a very young Class 0 protostar.

High-Mass Star Formation. I. The Mass Distribution of Submillimeter Clumps in NGC 7538We present submillimeter continuum maps at 450 and 850 μm of a12'×8' region of the NGC 7538 high-massstar-forming region, made using the Submillimeter Common-User BolometerArray (SCUBA) on the James Clerk Maxwell Telescope. We used an automatedclump-finding algorithm to identify 67 clumps in the 450 μm image and77 in the 850 μm image. Contrary to previous studies, we find apositive correlation between high spectral index, α, and highsubmillimeter flux, with the difference being accounted for by differenttreatments of the error beam. We interpret the higher spectral index atsubmillimeter peaks as a reflection of elevated dust temperature,particularly when there is an embedded infrared source, although it mayalso reflect changing dust properties. The clump mass-radiusrelationship is well fitted by a power law of the form M~R-xwith x=1.5-2.1, consistent with theories of turbulently supportedclumps. According to our most reliable analysis, the high-mass end(~100-2700 Msolar) of the submillimeter clump mass functionin NGC 7538 follows a Salpeter-like power law with index 2.0+/-0.3. Thisresult agrees well with similar studies of lower mass regions ρ Ophand Orion B. We interpret the apparent invariance of the shape of theclump mass function over a broad range of parent cloud masses asevidence for the self-similarity of the physical processes thatdetermine it. This result is consistent with models that suggest thatturbulent fragmentation, acting at early times, is sufficient to set theclump mass function.

Observations of Massive Star-Forming Regions with Water Masers: Mid-Infrared ImagingWe present here a mid-infrared imaging survey of 26 sites of water maseremission. Observations were obtained at the Infrared Telescope Facility3 m telescope with the University of Florida mid-infraredimager/spectrometer OSCIR, and the JPL mid-infrared camera MIRLIN. Themain purpose of the survey was to explore the relationship between watermasers and the massive star formation process. It is generally believedthat water masers predominantly trace outflows and embedded massivestellar objects, but may also exist in circumstellar disks around youngstars. We investigate each of these possibilities in light of ourmid-infrared imaging. We find that mid-infrared emission seems to bemore closely associated with water and OH maser emission than cm radiocontinuum emission from UC H II regions. We also find from the sample ofsources in our survey that, like groups of methanol masers, both waterand OH masers have a proclivity for grouping into linear or elongateddistributions. We conclude that the vast majority of linearlydistributed masers are not tracing circumstellar disks, but outflows andshocks instead.

The Outburst of V1647 Orionis Revealed by SpitzerWe present Spitzer Space Telescope observations of V1647 Ori, theoutbursting source lighting McNeil's Nebula, taken near the optical peakof the outburst in early March 2004. The source is easily detected inall Spitzer imaging bands from 3.6 to 70 μm. The fluxes at allwavelengths are roughly a factor of 15 brighter than pre-outburstlevels; we measure a bolometric luminosity of 44 Lsolar. Weposit that this event is due to an increase in the accretion luminosityof the source. Simple models of an accretion disk plus a tenuousenvelope can qualitatively explain the observed pre- and post-outburstspectral energy distributions. The accretion activity implied by ourresults indicates that the outburst may be intermediate between FUor-and EXor-type events. We also report the discovery of a previouslyunknown mid-infrared counterpart to the nearby Herbig-Haro object HH 22.

The CIDA Variability Survey of Orion OB1. I. The Low-Mass Population of Ori OB1a and 1bWe present results of a large-scale, multiepoch optical survey of theOrion OB1 association, carried out with the QUEST camera at theVenezuela National Astronomical Observatory. We identify for the firsttime the widely spread low-mass, young population in the Ori OB1a andOB1b subassociations. Candidate members were picked up by theirvariability in the V band and position in color-magnitude diagrams. Weobtained spectra to confirm membership. In a region spanning ~68deg2, we found 197 new young stars; of these, 56 are locatedin the Ori OB1a subassociation and 141 in Ori OB1b. The spatialdistribution of the low-mass young stars is spatially coincident withthat of the high-mass members but suggests a much sharper edge to theassociation. Comparison with the spatial extent of molecular gas andextinction maps indicates that the subassociation Ori OB1b isconcentrated within a ringlike structure of radius ~2°(~15 pc at 440pc), centered roughly on the star ɛ Ori in the Orion belt. The ringis apparent in 13CO and corresponds to a region with anextinction AV>=1. The stars exhibiting strong Hαemission, an indicator of active accretion, are found along this ring,whereas the center is populated with weak Hα-emitting stars. Incontrast, Ori OB1a is located in a region devoid of gas and dust. Weidentify a grouping of stars within a ~3 deg2 area located inOri OB1a, roughly clustered around the B2 star 25 Ori. The Herbig Ae/Bestar V346 Ori is also associated with this grouping, which could be anolder analog of σ Ori. Using several sets of evolutionary tracks,we find an age of 7-10 Myr for Ori OB1a and of ~4-6 Myr for Ori OB1b,consistent with previous estimates from OB stars. Indicators such as theequivalent width of Hα and near-IR excesses show that the numberof accreting low-mass stars decreases sharply between Ori OB1b and OriOB1a. These results indicate that although a substantial fraction ofaccreting disks remain at ages ~5 Myr, inner disks are essentiallydissipated by 10 Myr.Based on observations obtained at the Llano del Hato NationalAstronomical Observatory of Venezuela, operated by Centro deInvestigaciones de Astronomía (CIDA) for the Ministerio deCiencia y Tecnología, and at the Fred Lawrence WhippleObservatory (FLWO) of the Smithsonian Institution.Based on observations obtained at the 3.5 m WIYN Telescope. The WIYNObservatory is a joint facility of the University of Wisconsin-Madison,Indiana University, Yale University, and the National Optical AstronomyObservatory (NOAO).

A uniform CO survey of the molecular clouds in Orion and MonocerosWe report the results of a new large scale survey of the Orion-Monoceroscomplex of molecular clouds made in the J = 1 -> 0 line of12CO with the Harvard-Smithsonian 1.2 m millimetre-wavetelescope. The survey consists of 52 288 uniformly spaced spectra thatcover an area of 432 deg2 on the sky and represent the mostsensitive large-scale survey of the region to date. Distances to theconstituent molecular clouds of the complex, estimated from an analysisof foreground and background stars, have provided information on thethree dimensional structure of the entire complex.

A Near-Infrared (JHK) Survey of the Vicinity of the H II Region NGC 7538: Evidence for a Young Embedded ClusterWe describe the results of two near-infrared (K-band) imaging surveysand a three-color (JHK) survey of the vicinity of NGC 7538. The limitingmagnitudes are K~=16.5 and 17.5 mag for the K-band surveys and K~=15 magfor the JHK survey. We identify more than 2000 and 9000 near-infrared(NIR) sources on the images of the two K-band surveys and 786 NIRsources in the JHK survey. From color-color diagrams, we derive areddening law for background stars and identify 238 stars with NIRexcesses. Contour maps indicate a high-density peak coincident with aconcentration of stars with NIR excesses. We identify this peak as ayoung embedded cluster and confirm this result with the K-bandluminosity function (KLF), color histograms, and color-magnitudediagrams. The center of the cluster is atR.A.=23h13m39s34,decl.=61deg29'18.9". The cluster radius is ~3',~2.5 pc for an adopted distance d~=2.8 kpc. For d=2.8 kpc and reddeningEJ-K=0.55 mag, the slope of the logarithmic KLF of thecluster, s~0.32+/-0.03, agrees well with previous results for L1630(s=0.34) and M17 (s=0.26) found by C. Lada, E. Lada, and coworkers.Observations reported here were obtained at the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona.

Collapse and Fragmentation in Finite SheetsWe present two-dimensional simulations of finite, self-gravitatinggaseous sheets. Unlike the case of infinite sheets, such configurationsdo not constitute equilibrium states but instead are subject to globalcollapse unless countered by pressure forces or rotation. The initialeffect of finite geometry is to promote concentrations of material atthe edges of the sheet. If the sheet is not perfectly circular,gravitational focusing results in enhanced concentrations of mass. Inthe second-most simple geometry, that of an elliptical outer boundary,the general result is collapse to a filamentary structure with thedensest concentrations of mass at the ends of the filament. We suggestthat these simple calculations have interesting implications for thegravitational evolution of overall molecular cloud structure,envisioning that such clouds might originate as roughly sheetlikesections of gas accumulated as a result of large-scale flows in thelocal interstellar medium. We show some examples of local clouds withoverall filamentary shape and denser concentrations of mass and starclusters near the ends of the overall extended structure, suggestive ofour simple ellipse collapse calculations. We suggest thatcluster-forming gas is often concentrated as a result of gravity actingon irregular boundaries; this mechanism can result in very rapid infallof gas, which may be of importance to the formation of massive stars.This picture suggests that much of the supersonic ``turbulence''observed in molecular clouds might be gravitationally generated. Ourresults may provide impetus for further theoretical explorations ofglobal gravitational effects in molecular clouds and their implicationsfor generating the substructure needed for fragmentation into stars andclusters.

Sloan Digital Sky Survey Imaging of Low Galactic Latitude Fields: Technical Summary and Data ReleaseThe Sloan Digital Sky Survey (SDSS) mosaic camera and telescope haveobtained five-band optical-wavelength imaging near the Galactic planeoutside of the nominal survey boundaries. These additional data wereobtained during commissioning and subsequent testing of the SDSSobserving system, and they provide unique wide-area imaging data inregions of high obscuration and star formation, including numerous youngstellar objects, Herbig-Haro objects, and young star clusters. Becausethese data are outside the survey regions in the Galactic caps, they arenot part of the standard SDSS data releases. This paper presents imagingdata for 832 square degrees of sky (including repeats), in thestar-forming regions of Orion, Taurus, and Cygnus. About 470deg2 are now released to the public, with the remainder tofollow at the time of SDSS Data Release 4. The public data in Orioninclude the star-forming region NGC 2068/NGC 2071/HH 24 and a large partof Barnard's loop.